In utero hematopoietic stem cell transplantation (IUT) offers the hope of curing a number of hematological diseases by generating a state of hematopoietic stem cells (HSCs), the levels of donor cell engraftment that can be achieved by IUT are low, limiting the use of this therapy. Our aim is to extend the use of IUT to the treatment of diseases such as sickle cell anemia.. To achieve the high levels of donor cell engraftment needed to treat this disease it is the goal of this proposal to engineer HSCs to have a proliferative advantage over normal HSCs. This proposal will test the hypothesis that introduction of the erythropoietin (EpoR) into HSCs will render these altered cells responsive to erythropoietin (EPO). This will result in the altered HSCs and their progeny having a proliferative advantage over normal progenitors. Truncated forms of EpoR (tEpoR) will also be tested. These tEpoR, having deletions in the negative regulatory region of their cytoplasmic domains, deliver stronger proliferative signals, than EpoR. Fetal HSCs will be used as targets since they offer proliferative advantages over adult cells and are, therefore, susceptible to transduction by retroviral vectors. Lentiviral vectors will also be tested for their capacity to modify fetal as well as postnatal sources of HSCs. The effects of introducing the EpoR genes on the proliferation and differentiation of HSCs will be determined using various in vitro culture systems. It is hypothesized that ectopic expression of either EpoR or tEpoR expression on HSCs can make these cells more competitive than their normal counterparts, modified HSCs will be tested against control HSCs in a mouse model of human fetal hematopoiesis. The in vivo model will also be used to test the effects of EPO administration on the expansion of the modified HSCs. The ability of experiments will further determine if making HSCs responsive to HPO will have detrimental effect on the long-term reconstituting- and multi- lineage potential of HSCs.